The present invention is directed to fuel burners, and, more particularly, to a flame retention device for a fuel burner for use in a combustion application.
It is well known that some sort of flame stabilizing device is required in most combustion applications. Numerous U.S. Patents describe mechanical flame retention devices for use, for example, in process heaters, gas turbine combustors, waste gas flares, jet engine afterburners, gas-fired appliances, power generators and chemical reactors.
For example, U.S. Pat. No. 4,082,495 (Lefebvre) teaches a flame retention head assembly for use in the air tube of a fuel burner having a fuel nozzle in the tube.
U.S. Pat. No. 4,445,339 (Davis, Jr. et al.) teaches a flameholder for a gas turbine combustor that includes a primary flameholder such as an elongated V-gutter extending across a main flow stream of gas within the combustor. Vortices are shed by the trailing edges of the flameholder.
U.S. Pat. No. 4,548,576 (Chesters) teaches an apparatus for the burning of combustible gas. The device includes a flare having a vertical pipe connected to a gas source and a pipe having a flame stabilizer. The flame stabilizer is a cylinder coaxial with and lying within the pipe where the length of the cylinder is at least ten times the radial distance between the inner circumference of the pipe and the outer circumference of the cylinder. The cylinder has a cone at the outlet of the pipe that diverges at an angle of twenty to forty degrees to the horizontal.
U.S. Pat. No. 5,179,832 (Barcza) discloses a flameholder construction for gas turbine engines. The flameholder includes an augmenter and a fuel spray ring. A circumferential gutter is located downstream of the spray ring and a circumferential shroud is located radially inside the gutter. A circumferential outer shroud is located radially outside the gutter. These shrouds are arranged to confine fuel from the spray ring as well as a portion of the airflow to the zone of the gutter.
U.S. Pat. No. 5,186,620 (Hollinshead) teaches an inshot gas burner nozzle having a flame retention insert that enhances flame stability and reduces noise. The insert includes a central opening, secondary openings of smaller diameter arranged circularly around the central insert and a plurality of restricted peripheral openings in the form of stepped notches. The nozzle further includes plenum chambers which have restricted outlets that create back pressure within the plenums to improve cross-ignition of adjacent nozzles.
U.S. Pat. No. 5,669,766 (Hufton) discloses a fossil fuel air burner nozzle that directs streams of mixed fossil fuel and air into a combustion chamber. The nozzle includes a primary nozzle having nested, coaxial passages connected to a common supply conduit for the receipt of a flow of mixed fossil fuel and air and wherein the outer one of the nested passages is provided at its inlet end with a wall which lies in a plane normal to its axis. The wall has apertures which are symmetrically spaced about the axis.
U.S. Pat. No. 5,951,768 (Hahn) discloses a method of stabilizing a strained flame in a stagnation flow reactor. By causing a highly strained flame to be divided into a large number of equal size segments, this invention stabilizes a highly strained flame that is on the verge of extinction. The flame stabilizer is an annular ring mounted coaxially and coplanar with a substrate and has a number of vertical pillars mounted on the top surface thereby increasing the number of vertical pillars mounted on the top surface. The number of azimuthal nodes into which the flame is divided is increased. The flame is thereby preserved in an asymmetric structure necessary for stability.
Unfortunately, all of these devices add significantly to the cost and complexity of the various burner apparatuses. In some applications, the devices must be made of expensive high temperature alloys to withstand the heat of both the nearby flame and radiation from the furnace.
Newer low polluting burners are limited by the flameholder design. The stabilization mechanism relies on flames that range from stoichiometric to fuel rich. NOx formation is promoted in this combustion regime. For example, in U.S. Pat. No. 4,160,640 (Maev), a vortex burner is described that attempts to stabilize the flame without mechanical flameholders by swirling the gas flow. However, these inventions also promote combustion in the stoichiometric regime leading to relatively high NOx formation.
The primary objective of the invention is to stabilize combustion without the aid of a mechanical flame retention device. It is a further objective of the device to provide such stability in a way that promotes low formation of pollutants, especially NOx. A still further objective of the invention is to accomplish the above objectives with an apparatus constructed from common, inexpensive materials.
A first embodiment of the present invention is directed to a xe2x80x9csymmetricxe2x80x9d device for stabilization of a flame in a combustion apparatus. The symmetric device includes a primary oxidant pipe having a primary oxidant pipe internal surface and a primary oxidant pipe forward end, and a fuel pipe having a fuel pipe internal surface, a fuel pipe external surface, and a fuel pipe forward end. The fuel pipe is disposed at least partially internal to the primary oxidant pipe. A hollow primary oxidant flow conduit is formed between the fuel pipe external surface and the primary oxidant pipe internal surface. The device also includes a secondary oxidant pipe having a secondary oxidant pipe external surface and a secondary oxidant pipe forward end. The secondary oxidant pipe is disposed at least partially internal to the fuel pipe. A hollow fuel flow conduit is formed between the secondary oxidant pipe external surface and the fuel pipe internal surface. The secondary oxidant pipe has an internal, secondary oxidant conduit. The device also includes a primary oxidant flow source for supplying oxidant at a first oxidant flow velocity to the primary oxidant conduit, a fuel flow source for supplying a fuel at a fuel flow velocity to the fuel flow conduit, and a secondary oxidant flow source for supplying oxidant at a second oxidant flow velocity to the secondary oxidant conduit. The first oxidant flow velocity is greater than the second oxidant flow velocity and the fuel flow velocity is less than the second oxidant flow velocity. The primary oxidant pipe end extends past the fuel pipe forward end and the fuel pipe forward end extends past the secondary oxidant pipe end. A mismatch in velocity between flowing fuel and flowing oxidant generates a large scale vortex of the oxidant and fuel as they mix.
Preferably, the first oxidant velocity is in a range from about 30 feet per second to about 90 feet per second, the fuel flow velocity is in a range from about 2 feet per second to about 6 feet per second, and the second oxidant flow velocity is in a range from about 15 feet per second to about 45 feet per second. Also, more broadly, the first oxidant flow velocity is preferably greater than 30 ft./sec., the fuel flow velocity is preferably less than 20 ft./sec. and the second oxidant flow velocity is preferably between the first oxidant velocity and the fuel flow velocity.
Optionally, the various pipes may have a round cross-sectional shape wherein the primary oxidant pipe end extends past the fuel pipe forward end by a first length and the fuel pipe forward end extends past the secondary oxidant pipe end by a second length, and wherein a ratio of the first length to the fuel pipe diameter is approximately 1 to 3, a ratio of the first length to the primary oxidant pipe diameter is approximately 1 to 3, and a ratio of the second length to the secondary oxidant pipe diameter is approximately 1 to 3.
A second embodiment of the present invention is directed to an xe2x80x9casymmetricxe2x80x9d device for stabilization of a flame in a combustion apparatus. The asymmetric device includes an oxidant pipe having an oxidant pipe internal surface and an oxidant pipe forward end, and a fuel pipe having a fuel pipe internal surface, a fuel pipe external surface, and a fuel pipe forward end. The fuel pipe is disposed at least partially internal to the oxidant pipe. A hollow oxidant flow conduit is formed between the fuel pipe external surface and the oxidant pipe internal surface. An oxidant feed pipe is connected to the oxidant pipe preferably at an angle to the oxidant pipe such that an oxidant flowing through the oxidant feed pipe is adapted to travel through the oxidant feed pipe and impinge on the fuel pipe external surface and then travel through the oxidant flow conduit to the fuel pipe forward end. An oxidant flow source is provided for supplying the oxidant at an oxidant flow velocity to the oxidant feed pipe. A fuel flow source is provided for supplying a fuel at a fuel flow velocity to the fuel pipe. The oxidant flow velocity is greater than the fuel flow velocity and the oxidant pipe forward end extends past the fuel pipe forward end. Again, a mismatch in velocity between flowing fuel and flowing oxidant generates a large scale vortex of the oxidant and fuel as they mix.
Preferably, the oxidant flow velocity is in a range from about 10 feet per second to about 50 feet per second, the fuel flow velocity is in a range from about 2 feet per second to about 10 feet per second, and the oxidant feed pipe is connected at an angle of about ninety degrees to the oxidant pipe.
Optionally, the various pipes of the device of the second embodiment may have a round cross-sectional shape wherein the oxidant pipe forward end extends past the fuel pipe forward end by a first length and the fuel pipe extends past an uppermost point of an internal surface of the oxidant feed pipe by a second length, wherein a ratio of the first length to the oxidant feed pipe diameter is approximately 0.5 to 2, wherein a ratio of the second length to the fuel pipe diameter is greater than or equal to about 1, and wherein a ratio of the oxidant pipe diameter to the fuel pipe diameter is approximately 1.2 to 1.8.